Size reduction of metal-oxide-semiconductor field-effect transistors (MOSFET), including reduction of the gate length and gate oxide thickness, has enabled the continued improvement in speed performance, density, and cost per unit function of integrated circuits over the past few decades. To enhance transistor performance further, strain may be introduced in the transistor channel for improving carrier mobilities. Therefore, strain-induced mobility enhancement is another way to improve transistor performance in addition to device scaling. There are several existing approaches of introducing strain in the transistor channel region.
In one conventional approach, as described in a paper by J. Welser et al., published at the December 1992 International Electron Devices Meeting held in San Francisco, Calif., pp. 1000-1002 and incorporated herein by reference, a relaxed silicon germanium (SiGe) buffer layer is provided beneath the channel region. In such a device, a semiconductor device includes a strained silicon layer formed over and abutting a relaxed SiGe layer, which is formed over and abutting a graded SiGe buffer layer.
The relaxed SiGe layer has a larger lattice constant compared to relaxed Si, and the thin layer of epitaxial Si grown on the relaxed SiGe will have its lattice stretched in the lateral direction, i.e., it will be under biaxial tensile strain. Therefore, a transistor formed on the epitaxial strained silicon layer will have a channel region that is under biaxial tensile strain. In this approach, the relaxed SiGe buffer layer can be thought of as a stressor that introduces strain in the channel region. The stressor, in this case, is placed below the transistor channel region.
Significant mobility enhancement has been reported for both electrons and holes in bulk transistors using a silicon channel under biaxial tensile strain. In the above-mentioned approach, the epitaxial silicon layer is strained before the formation of the transistor. But there are concerns about the strain relaxation upon subsequent CMOS processing where high temperatures are used. In addition, this approach is very expensive since a SiGe buffer layer with thickness in the order of micrometers has to be grown. Numerous dislocations in the relaxed SiGe buffer layer exist and some of these dislocations propagate to the strained silicon layer, resulting in a substrate with high defect density. Thus, this approach has limitations that are related to cost and fundamental material properties.
In another approach, strain in the channel is introduced after the transistor is formed. In this approach, a high stress film is formed over a completed transistor structure formed in a silicon substrate. The high stress film or stressor exerts significant influence on the channel, modifying the silicon lattice spacing in the channel region, and thus introducing strain in the channel region. In this case, the stressor is placed above the completed transistor structure. This scheme is described in detail in a paper by A. Shimizu et al., entitled “Local mechanical stress control (LMC): a new technique for CMOS performance enhancement,” published in pp. 433-436 of the Digest of Technical Papers of the 2001 International Electron Device Meeting, which is incorporated herein by reference.
The strain contributed by the high stress film is believed to be uniaxial in nature with a direction parallel to the source-to-drain direction. However, uniaxial tensile strain degrades hole mobility while uniaxial compressive strain degrades the electron mobility. Ion implantation of germanium can be used to selectively relax the strain so that the hole or electron mobility is not degraded, but this is difficult to implement due to the close proximity of the n and p-channel transistors.
Accordingly, what is needed in the art is an improved transistor and method thereof that addresses the above-discussed issues.